[SSAF][PointerFlowExtractor] Handle empty initializer lists for scalars and unions (#201968)
Empty initializer lists for scalars and unions are corner cases that
were previously missed by the PointerFlowExtractor. This commit adds
support for these cases. Because the initializing values are implicitly
constant 0 (or nullptr), they do not bridge entity pointer levels.
rdar://178856689
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Co-authored-by: Balázs Benics <benicsbalazs@gmail.com>